Thermalctld APIs for recording min and max temp (#131)

sonic-platform-base: Changes to introduce APIs for modular chassis for thermalctld

HLD: sonic-net/SONiC#646

Introducing thermal APIs to get min and max temperatures of each sensors
  - get_minimum_recorded()
  - get_maximum_recorded()

sonic_platform_base/thermal_base.py

@@ -92,3 +92,23 @@ def get_low_critical_threshold(self):
             up to nearest thousandth of one degree Celsius, e.g. 30.125
         """
         raise NotImplementedError
+
+    def get_minimum_recorded(self):
+        """
+        Retrieves the minimum recorded temperature of thermal
+
+        Returns:
+            A float number, the minimum recorded temperature of thermal in Celsius
+            up to nearest thousandth of one degree Celsius, e.g. 30.125
+        """
+        raise NotImplementedError
+
+    def get_maximum_recorded(self):
+        """
+        Retrieves the maximum recorded temperature of thermal
+
+        Returns:
+            A float number, the maximum recorded temperature of thermal in Celsius
+            up to nearest thousandth of one degree Celsius, e.g. 30.125
+        """
+        raise NotImplementedError